1. Field of the Invention
The invention relates to subscriber interfaces to telephone lines, in general, and in particular to a subscriber line interface circuit (SLIC). The invention provides a method and apparatus for varying the DC feed characteristics in a SLIC coupled to a telephone subscriber line. More particularly, the invention relates to a method and apparatus for actively varying the DC feed characteristic of the SLIC in response to telephone operating conditions.
2. Related Art
Subscriber line interface circuits (SLICs) were developed to provide an interface between a telephone subscriber and a telephone central office. One such SLIC is the Am7953, manufactured by Advanced Micro Devices, Inc. of Sunnyvale, Calif. When used with other devices, such as a CODEC/filter or a subscriber line audio processing circuit (SLAC), the SLIC can be programmed to realize desired telephone service characteristics. Together, the SLIC and the other circuitry, such as the SLAC, can be implemented as part of a line card to achieve battery feed, overload protection, ringing, line supervision, 2-wire to 4-wire analog hybrid conversion, and test features necessary for basic telephone service. A telephone line consists of two conductors labelled tip or A and ring or B, respectively. Accordingly, the SLIC has tip and ring terminals for connection to the telephone line.
In a telephone system, power is fed from a central office to its various subscribers by subscriber loops. The subscriber equipment, such as a telephone, must receive sufficient electric current to properly function. The electric circuit in the central office supplying the current of the load formed by the transmission line and the subscriber equipment is known as a battery feed circuit.
The battery feed circuit supplies a DC current to the subscriber. Upon the DC current are superimposed AC signals, typically of audio frequency, by which information is conveyed to the subscriber from the central office and from the subscriber to the central office and onward.
The typical battery voltage in the United States is -48 volts. One conventional battery feed circuit such as that shown in FIG. 1a has a battery feed characteristic such as that shown in FIG. 1b. As shown in FIG. 1b, as the load current increases from 0 to 120 milliamps, the battery feed voltage decreases from 48 volts to 0 volts. The battery feed characteristic of a conventional battery feed circuit is illustrated in FIG. 1b by line 1 in which V.sub.TR =48 V-(I.sub.L .times.400 .OMEGA.).
More contemporary systems using subscriber line interface circuits rarely require more than 40 to 60 milliamps and therefore implement current limiting, for example, at 60 milliamps, as shown in FIG. 1b. Current limiting reduces the overall power dissipation of the SLIC and of the subscriber loop. Electronic SLICs use a portion of the battery voltage to drive amplifiers within the SLICs, thereby resulting in an overhead voltage loss. As a result of this overhead voltage loss, without additional circuitry, an electronic SLIC would be unable to supply adequate current to telephone lines having long loops.
An electronic SLIC can emulate the feed resistance without using a physical resistor, thereby avoiding the power dissipation and voltage drops in a physical resistor. However, a low value sense resistor is usually required as a means for monitoring telephone line current. When the telephone line voltage approaches the battery voltage, the electronic circuit can reduce the emulated feed resistance, thus reducing the effective voltage drop required to support the feed characteristic. However, it is impractical to reduce the emulated feed resistance to a value as low as the sense resistors. Therefore, there still remains overhead voltage lost to the emulated impedance. One purpose of the invention described herein is to recover most of this overhead voltage lost to the emulated impedance.
FIG. 2 shows a battery feed characteristic for an electronic constant current SLIC. The output amplifiers of the SLIC introduce an offset voltage, for example, five volts of overhead voltage. Thus, in the on-hook condition (zero load current), the voltage across the tip and ring terminals is approximately the battery voltage reduced by five volts. In the load condition, after the telephone is off hook, the battery feed voltage is reduced by another 4 volts to the battery voltage minus 9 volts. The additional 4 volt reduction is a result of the emulated resistance in this region and is much greater than necessary for proper device operation. Therefore, it is desirable to reduce this additional voltage and to provide higher voltage in the off hook condition.